The present invention is directed to high frequency power electronics, and more particularly to a power conversion system with a galvanically isolated high frequency link.
The low power density of conventional electrical systems has been a significant barrier to the deployment of ‘more electric’ systems for particular classes of ships. Improvements in power densities that are achieved by advanced turbines and generators are often diluted by the need for bulky ancillary equipment, such as transformers.
Many modern power systems require large and heavy 50 Hz or 60 Hz conventional transformers. The weight and volume of these transformers is a major barrier to the development of expanded electrical capabilities associated with certain future power system applications.
High frequency “solid state transformers”, which are enhanced replacements of bulky line frequency iron core transformers have drawn considerable interest for utility distribution systems and locomotive applications. The Intelligent Universal Transformer (IUT) program launched by the Electric Power Research Institute (EPRI), and medium frequency transformer prototype demonstrations by Bombardier, ABB, and Deutsche Bahn for locomotive application, represent examples of state-of-the-art research in this area.
Architectures proposed for these electronic transformers have centered on using cascaded converter blocks or multi-level neutral point clamped (NPC) converters to handle the high voltages on the primary side.
FIG. 1 exemplifies the large number of cells, or levels, required in the cascaded converter block architectures. These architectures are disadvantageous in that they inherently necessitate a high level of complexity and part count. A large number of cascaded cells are required with this approach due to limited voltage ratings of available silicon (Si) semiconductors.
FIG. 2 exemplifies the large number of cells, or levels, required in a multi-level NPC converter block architecture. The architecture illustrated in FIG. 2 utilizes a HV-IGBT-based multi-level NPC converter configuration on the primary side. The limited voltage rating and switching frequency of current high voltage IGBTs result in a large component count and low system performance.
High power density solid-state electronics transformers for solid-state power substations (SSPS) provide functionalities beyond a conventional line frequency iron core transformer. These functionalities include: (1) step up or down voltage level with galvanic isolation between low frequency input and output, which is the function of a conventional line frequency transformer, with a much higher power density resulted from intermediate high frequency isolation transformer; (2) ability to convert frequency, e.g. get DC or 60 Hz or 400 Hz power at the output from 50 Hz or 60 Hz input power; (3) generate multiple outputs at different frequencies and voltage levels; and (4) provide advanced control functions for entire power system, such as reactive power compensation, voltage regulation, and active harmonic filtering, active damping etc.
Emerging silicon carbide (SiC) devices, e.g. SiC MOSFET, SiC IGBT, SiC Schottky, PiN and JBS diodes, etc. offer application benefits, such as lower conduction and switching losses, higher voltage and higher temperature capabilities than their counterparts of Si devices. Those features are critical to enable high density high frequency, medium voltage SSPS. However, at present state those SiC devices have a manufacturing yield that is lower than Si devices, and a cost that is higher than Si devices. Significant challenges remain to developing such a smaller solid-state electronics transformer for a solid-state power substation (SSPS). These include, but are not limited to:
Conventional high power converter topologies, such as multi-level NPC converters, present significant design challenges at high frequencies due to complex device interconnections and packaging. Parasitic inductances can lead to increased electrical stresses and degradation of performance;
Thermal management of high frequency transformers is a major challenge due to their reduced size;
Passive components, such as DC bus capacitors, input and output filters, and contactors can limit power densities. Minimizing the use of such devices is critical;
Multiple cascaded power conversion stages can reduce the SSPS efficiency; and
Device count should be minimized to account for yield constraints of early SiC devices.
In view of the foregoing, it would be both advantageous and beneficial to provide a fault tolerant, high frequency link-based transformer (i.e. solid-state power substation (SSPS)) including controls to minimize all passive components associated with the SSPS. It would be further beneficial if the SSPS architecture could be implemented using well-understood power blocks with a parts count that is lower than that achievable when using known electronic transformer architectures.